Infrared spectroscopy (IR spectroscopy) is a useful analytical technique for analyzing organic molecules. In this article, we will explore the science behind IR spec, the equipment, and some techniques for elucidating the structure of organic molecules using an infrared spectrum.
Topics Covered in Other Articles
- Organic Structure Elucidation Using NMR and IR
- Covalent Bonding
- Organic Functional Groups
- Scientific Method
Principles of IR Spectroscopy
What is Infrared Light
Light consists of individual particles, called photons, which move in a wave like pattern. The length of these waves determines the color we see.
Humans can only see a small portion of these wavelengths, from 400 to 700 nanometers. However, just beyond red light is infrared light (>700nm). Human eyes are not able to see infrared light, but it exists just as much as the colors we see.
Upon interaction, objects absorb incident light. This contributes to the colors we see (or don’t see). Plants look green because they absorb red and blue light while reflecting the green back towards our eyes. Objects that appear black absorb all visible light, likewise, objects that appear white reflect all visible light. It can be very useful to analyze the amount of light absorbed and reflected by objects to gain insight into the molecules and properties present. Organic molecules absorb plenty of infrared light (but typically not much visible light because most organic compounds are white solids), making them particularly suited for analysis in the UV spectrum.
The Infrared Spectrophotometer
Infrared light is emitted from an emitter and split. The beams then pass through a sample and a blank reference. The differences in light are compared by a detector, and the results are interpreted by a computer into the familiar IR spectrum.
Fourier Transform IR Spectroscopy
Fourier Transform Infrared Spectroscopy (FT-IR) is an analytical technique that measures the same data as traditional infrared spectrometers. However, the instrumentation is completely different. A set of optics collimates and splits a beam almost evenly over the IR frequencies. These split beams recombine to form a unique spectrum of IR light that passes through the samples. Importantly, the FT-IR instrument shifts a mirror involved in the beam splitting, altering the light’s path length, which produces a unique spectrum of IR light at each mirror position. Computerization can then produce a standard IR spectrum from the path length and interference data using a function known as a Fourier transform.
FT-IR has the advantage of being much quicker for high throughput applications (such as for detection of samples from liquid chromatography (LC-IR)), but usually sacrifices some resolution compared to the traditional instrumentation.
An IR Spectrum
After the computer module has received the data from the photodetectors for both the reference path and the sample, the computer calculates the amount of each frequency of photon along the detector. By dividing the amount of photons of a given frequency for the sample by the amount of photons from the reference, the percent absorption can be calculated. This data is then traditionally graphed with the percent absorption on the vertical axis and the frequency (recorded as cm-1 (the number of oscillations of the photon in a centimeter)) decreasing from left to right on the horizontal.
Infrared radiation is ideal for analysis of organic molecules because covalent chemical bonds absorb this energy and produce specific movements in response. IR spectroscopy studies two main classes of vibrational modes, stretching and bending.
IR Spectroscopy: Stretching
The most common form of infrared absorption is stretching. Each bond has a particular frequency at which the atoms resonate towards and away from each other. When exposed to this exact frequency of infrared light, the bond between the atoms absorbs the light and uses the energy to increase the amplitude of the stretching. All covalent bonds will exhibit stretching in the IR region, however, some are much more easy to detect. O-H stretching, C-H stretching, and C=O stretching are some of the easiest peaks to detect on a spectrum. Stretching bonds usually requires a lot of energy, and thus usually yield peaks with a higher frequency (wavenumber) on a spectrum. An important principle of IR stretching is that the atoms will move to conserve their center of mass while they stretch.
Stretching of two geminal bonds usually leads to a sharp peak, can can occur in two different vibrational modes. Symmetric stretching is usually observed at higher frequencies than asymmetric stretching.
IR Spectroscopy: Bending
Molecules also absorb energy into their bonds by means of bending. Bending bonds usually requires less energy than stretching, and thus usually occurs in the lower frequencies of IR spectra. This bending can happen in a diverse array of modes, and contributes to the many signals in the lower frequencies of IR spectra. Often times, assigning these lower frequencies is harder, and creates the ‘fingerprint region’ at frequencies less than 1300 cm-1.
The Fingerprint Region
At frequencies less than 1300 cm-1, IR spectra tend to have a lot of sharp peaks. Assigning specific functional groups to these peaks is often challenging due to the number of signals at or near each value. However, there tend to be some easily identifiable peaks (such as a bending alkene near 975) which can corroborate information from the more readable part of the spectrum. The fingerprint region is unique for each molecule, and software uses it to compare the chemical identity of two substances from their spectra.
Factors Affecting IR Absorption
While the frequencies of absorption are well studied, there are some effects that alter the observed frequency on the spectrum. It is important to keep this in mind when conducting IR spectroscopy to elucidate molecular structure.
Conjugation occurs when the pi electrons of one functional group overlap with those of another, creating a delocalized pi system (like benzene). This lowers the frequency about 10-50 cm-1. Two double bonds separated by a single bond is a hallmark of conjugation. This usually is a conjugated system, and will not exhibit typical IR absorption. Benzoic acid, as an example, is shown below.
Hybridization also lowers frequency. sp3 atoms tend to have absorb higher frequency IR light that their sp2 or sp counterparts. However, the conjugation trend is more consistent, and the presence of differently hybridized functional groups needs corroboration with other information from the spectrum.
IR Spectroscopy Table
|IR Absorption Range (cm-1)
|Amine (Tert. or Quat.)
IR Spectroscopy Example: ethyl prop-2-ynoate
- 3275: Alkyne C-H Stretching, Indicates Terminal Alkyne
- 2990: Alkene C-H Stretching
- 2130: CΞC Stretching: Frequency Lowered Due to Conjugation of the Alkyne Pi Electrons with the Carbonyl Pi Electrons
- 1225: C-O Stretching: Ester
- 1030: C-O Stretching
- 760: C-H Bending: Alkane
IR Spectroscopy Practice Problems
Match the Following Molecules With Its Spectrum: